Abstract
Liquid surface oceans are a seemingly unique feature of Earth. Long-term, global sea level depends on the balance of water fluxes between Earth's mantle and surface: between mantle degassing through volcanism and mantle regassing via the subduction of hydrous minerals. However, the overall balance of these fluxes at geological timescales remains uncertain. Geological observations suggest the stability of the long-term sea level and thus a near-steady-state regassing-degassing balance. In contrast, according to current thermopetrological modelling, the global input of H2O inferred from geophysical observations leads to an unequivocal excess of regassing relative to degassing. Here we use recent experimental high-pressure data on natural hydrated peridotites to update the thermopetrological models and to reassess the calculations of H2O fluxes into the mantle via subduction. Our models of 56 subduction transects show that a global input of 15−20 × 108 TgH2O every million years yields a limited global mantle regassing of 2.0−3.5 × 108 TgH2O every million years. The regassing occurs exclusively via the hydrated lithospheric mantle of the coldest subducting plates. Our requantification of the H2O budget associated with subduction matches the estimations of mantle degassing and suggests that global sea levels have been relatively stable over geological timescales.